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The finding frustrates the search for an effective way to prevent preeclampsia, or dangerously high blood pressure in pregnancy, in women who are at high risk for it.
"We thought we had something that would help so many women, but at the same time we can never be disappointed with the truth," said senior study author Dr. Mark Walker. He's head of obstetrics, gynecology and newborn care at The Ottawa Hospital in Ontario, Canada.
Preeclampsia is the leading cause of pregnancy-related complications and death worldwide, affecting about 10 million women a year. The most effective treatment is to deliver the baby, sometimes preterm. Medications, such as those to lower blood pressure, are also recommended for some women.
Folic acid is a B vitamin that helps cells grow. It is recommended for women who are trying to conceive and those who are in their first trimester of pregnancy, to prevent neural tube birth defects such as spina bifida.
Previous research, though inconsistent, suggested high doses throughout pregnancy might also offer some protection against preeclampsia. But the new study found no benefit from a high dose after the first trimester.
"We know many, many women who are being treated that don't need to be so, if anything, our results are going to be changing practice," said Walker.
The study included about 2,460 women in five countries who had at least one risk factor for preeclampsia -- obesity, twin pregnancy, existing high blood pressure, pre-pregnancy diabetes or a previous history of preeclampsia.
Each received up to 1.1 milligrams of folic acid daily throughout their pregnancy. About half were randomly chosen to also take a daily high-dose (4-milligram) supplement, starting at 8 to 16 weeks of pregnancy. The others took a placebo.
About 15 percent of the women in the high-dose group developed preeclampsia, as did 13.5 percent of those who took the dummy pills -- not a meaningful difference.
Rates of stillbirth were also similar between the two groups, between 1 percent and 2 percent. In addition, the study found no difference between the groups for any other negative outcomes.
The results were published Sept. 13 in The BMJ.
Dr. Aaron Caughey, associate dean for Women's Health Research and Policy at Oregon Health and Science University, reviewed the findings.
"We've been looking at so many things, but nothing seems to work," he said. "I don't find it particularly surprising. You hate to conduct a large trial like this and have negative findings, but the point is to find out if something works or not."
Doctors routinely prescribe a baby aspirin for women who are at high risk of preeclampsia, Caughey said, and an increasing number are recommending a double dose. He said a recent study of the diabetes drug metformin in pregnant women also found a small reduction in preeclampsia, but further investigation is needed.
Walker noted that his team plans to follow the babies born to mothers in the study for six years, to see if the higher doses of folic acid affected their health or brain development.
"After this study, we are even hungrier than we were 10 years ago to find other therapies," Walker said.
He noted that preeclampsia may have genetic causes in some patients, while environmental exposure or immune system issues may be at play in others. Ultimately, he suspects the solution will be found in precision medicine -- therapies tailored to individual patients.
Dr. Lucy Chappell, a research professor in obstetrics at King's College London, co-wrote an editorial that accompanied the study. It called the findings disappointing, noting that other promising treatments, including antioxidant supplements, have also failed to yield hoped-for benefits.
"All pregnant women and their families hope for a healthy pregnancy and a happy outcome; until we find additional ways to prevent preeclampsia, thousands of women each year will not achieve this goal," Chappell and her colleagues wrote.
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SOURCES: Mark Walker, M.D., M.Sc., chief, obstetrics, gynecology and newborn care, Ottawa Hospital, and chairman and professor, obstetrics and gynecology, University of Ottawa, Ontario, Canada; Aaron Caughey, M.D., Ph.D., chairman, obstetrics and gynecology, Oregon Health and Science University, and associate dean, women's health research and policy, OHSU School of Medicine, Portland; Sept. 13, 2018, BMJ